It is desirable, in medical practice, to provide hypodermic syringes with an aspirating capability so as to enable the medical practitioner, after insertion of the hypodermic needle into the injection site, to determine whether the needle has entered a major blood vessel and, depending upon whether blood is drawn back into the syringe during aspiration, and depending also upon the desired route of administration, to either proceed with the injection or to withdraw the needle and relocate it before injection as appropriate to the circumstances.
In general, aspiration in hypodermic syringes can be achieved by adapting the syringe holder for either manual or self-aspiration, depending upon whether aspiration is achieved by generation of a slight negative pressure in the syringe holder by the slight manual withdrawal of the ampoule piston or whether the slight negative pressure is generated by some mechanical action effected by the particular syringe design.
Self-aspirating syringes are often complex in structure and are therefore usually expensive to manufacture. Manually operable aspirating syringes have the potential for relative simplicity of construction, because those used with disposable medicament-containing ampoules require only a means to securely hold the ampoule in the syringe holder barrel to prevent axial displacement thereof in the holder and a means to affix the end of the plunger rod of the holder to a slidable piston closing the inner end of the ampoule. With such positive engagement between the plunger and the piston, slight withdrawal of the interconnected plunger/piston generates the essential negative pressure in the ampoule required for aspiration.
U.S. Pat. No. 4,585,445 issued Apr. 29,1986 to F. B. Hadtke describes hypodermic syringe holders, which have achieved widespread commercial acceptance, for use in combination with disposable ampoules. The holders immobilize a cartridge ampoule within the holder during use and provide manual aspirating capability. As indicated by FIG. 2 therein, the holder comprises five working parts, i.e., a body portion, a clamping element, a plunger element, a piston engaging means and a boss element. The boss serves to hold all the elements of the syringe holder together and is affixed to the body portion of the holder mechanically, by gluing, or by thermal, solvent or sonic welding. A significant problem with the holder described in U.S. Pat. No. 4,585,445 is that the boss element tends to pop off the holder during actuation, rendering the holder useless. Yet another problem with such holder is that during the welding process, the boss element can get inadvertently welded to the clamping element. Moreover, it is apparent that it would be desirable to provide a syringe holder which accomplishes the objectives described in U.S. Pat. No. 4,585,445 with fewer working parts and which is less expensive to manufacture.